Curved cuts within a transparent material are generated, in particular, in laser-surgical methods, especially in ophthalmic surgery. This involves focusing treatment laser radiation within the tissue, i.e. beneath the tissue surface, so that optical breakthroughs are generated in the tissue.
In the tissue, several processes initiated by the laser radiation occur in a time sequence. If the power density of the radiation exceeds a threshold value, an optical breakthrough will result, generating a plasma bubble in the material. After the optical breakthrough has formed, said plasma bubble grows due to expanding gases. If the optical breakthrough is not maintained, the gas generated in the plasma bubble is absorbed by the surrounding material, and the bubble disappears again. However, this process takes very much longer than the forming of the bubble itself. If a plasma is generated at a material boundary, which may quite well be located within a material structure as well, material will be removed from said boundary. This is then referred to as photo ablation. In connection with a plasma bubble which separates material layers that were previously connected, one usually speaks of photo disruption. For the sake of simplicity, all such processes are summarized here by the term optical breakthrough, i.e. said term includes not only the actual optical breakthrough, but also the effects resulting therefrom in the material.
For a high accuracy of a laser surgery method, it is indispensable to guarantee high localization of the effect of the laser beams and to avoid collateral damage to adjacent tissue as far as possible. It is, therefore, common in the prior art to apply the laser radiation in a pulsed form, so that the threshold value for the power density of the laser radiation required to cause an optical breakthrough is exceeded only during the individual pulses. In this regard, U.S. Pat. No. 5,984,916 clearly shows that the spatial extension of the optical breakthrough (in this case, of the generated interaction) strongly depends on the pulse duration. Therefore, precise focusing of the laser beam in combination with very short pulses allows the placement of the optical breakthrough in a material with great point accuracy.
The use of pulsed laser radiation has recently become established practice particularly for laser-surgical correction of visual defects in ophthalmology. Visual defects of the eye often result from the fact that the refractive properties of the cornea and of the lens do not cause orderly focusing on the retina.
U.S. Pat. No. 5,984,916 mentioned above, as well as U.S. Pat. No. 6,110,166, describe methods of the above-mentioned type for producing cuts by means of suitable generation of optical breakthroughs, so that, ultimately, the refractive properties of the cornea are selectively influenced. A multitude of optical breakthroughs are joined such that a lens-shaped partial volume is isolated within the cornea of the eye. The lens-shaped partial volume which is separated from the remaining corneal tissue is then removed from the cornea through a laterally opening cut. The shape of the partial volume is selected such that, following removal, the refractive properties of the cornea are modified so as to cause the desired correction of visual defect. The cuts required here are curved, which makes three-dimensional shifting of the focus necessary. Therefore, a two-dimensional deflection of the laser radiation is usually combined with a simultaneous focus shift.
In order to isolate the partial volume, it is indispensable, of course, to generate the optical breakthroughs at predetermined locations. In doing so, the quality of the generated cut depends on the uniformity of the arrangement of the optical breakthroughs. This applies, in particular, to the aforementioned ophthalmic operations effecting a refractive correction, because here, the quality of the cut is inseparably connected with the optical quality of the result achieved.
In order to produce the curved cuts with high quality, it is therefore indispensable to arrange the optical breakthroughs in series with a high density. However, this is detrimental to quick production of a cut for two reasons: On the one hand, the time required for generating the cut increases as the required number of optical breakthroughs increases. On the other hand, a tight sequential arrangement of optical breakthroughs requires waiting after each breakthrough until the amount of gas generated in the plasma bubble has been re-absorbed by the surrounding tissue, before the next optical breakthrough can be generated immediately adjacent thereto. Otherwise, the optical breakthrough could not be generated with sufficient safety, because the laser radiation would possibly be focused into a still existing plasma bubble and would not cause an optical breakthrough there.
Therefore, it is an object of the invention to improve a method and an apparatus for producing curved cuts as mentioned above, so that a good quality of the optical cut surface is possible while at the same time forming the cut as quickly as possible.